Means for determining the effectiveness of sound reducing means



INVENTOR Donald 1 Mp Zne WM ATTORNEYS wmw m nw J ly 22, 1958 D. Y. MILNE MEANS FOR DETERMINING THE EFFECTIVENESS 0F SQUND REDUCING MEANS Filed Feb. 17, 1955 i PL wN nw NH II N United States Patent MEANS FOR DETERlt/HNING THE EFFECTIVE- NESS F SGUND REDUCING MEANS Donald Y. Milne, Homewood, 13L, :assignor to Maremont Automotive Products, Inc., Chicago, Ill., a corporation of Illinois Application February 17, 1955, Serial No. 488,880

4 Claims. (Cl. 181-.5)

This invention relates to a method and apparatus for defining the sound energy decibel output of a given engine, e. g., internal combusion, diesel, etc., and at any frequency (cycles per second).

The primary object of the invention is to provide a method and means for defining the sound energy decibel output at any frequency before and after the sound has passed through a sound reducing means, e. g., an automobile exhaustmuffier. It is possible by reason of this invention to make a comparison of the sound a as it leaves any engine at any frequency as against the sound b as it leaves the mufiler. To determine the extent of the sound removed by the muffler, one has only to ascertain the value of a-b. As a result of this invention, engineers will now be able to design mufliers and other sound-absorbing or reducing equipment properly and with a certainty not heretofore possible.

The accompanying drawing is a diagrammatic illustration of the invention, it being understood that, as far as the specific equipment employed is concerned, this is conventional and available commercially in numerous types on the open market, and may be obtained from many sources.

The numeral indicates a suitable audio oscillator,

the frequency of which can be varied, e. g., from about 20 cycles per second to 20,000 cycles per second, to create different sound outputs analogous to the frequency of the noises created respectively by various engines with which mufflers are to be associated. This is preferable to obtaining the sound effect by providing the specific engine in each case with microphone equipment.

Associated with the oscillator 10 is a standard type amplifier 11 and a loud speaker 12 mounted in a heavy wooden cabinet from which the noise is piped at 13 to a mufller 14. Disposed in the pipe 14 on the inlet side of the muffier is an adjustably and detachably connected pipe section 15 which is slidably, telescopically mounted in and between the adjacent pipe sections 13a of the loudspeaker and 13b on the input side of the muffler, as shown at 16.

Associated with the slidably mounted piping section 15 is an input microphone 17 to pick up the noise before it enters the muffier, this microphone being connected to a suitable sound level meter 18 which gives an input reading in decibels on the visible indicator 19.

The sound passes through the mufiier 14 on the outlet side of which is a pipe 20 terminating in a conventional anti-reflection unit 21 whereby the sound is removed and never returns to the system.

Disposed in the pipe 20 on the outlet side of the mufller is an output microphone 22 connected to a sound level meter 23 having means 24 for indicating in decibels the output amplitude of the sound after it has been subjected to the action of the muffler.

From the foregoing it will be observed that the sound created by the audiocscillator 10 or other source, such as the actual engine, is-amplified to increase its loudness for I by the oscillator.

transmission by the loudspeaker in the pipe 1345 where it is picked up by 'the input microphone 17 and the amplitude :of the noise is recorded *by the input sound level meter 18. The sound travels through the mufiler 14 which reduces the sound level in accordance with 'its capacity to absorb the noise at the frequency produced The rremaining .sound passing through the pipe 20 on the output side of the mufiier is picked up by the output microphone 22 and recorded on the output sound level meter 23. The remaining sound passes through the anti-reflection unit 21 and out of the system.

The difference between the input and output readings U and 24, respectively, gives a value in decibels of the muflling efiiciency of the particular mufiler 14 being tested at any frequency setting. A chart of these values over the frequency span desired, gives a curve representing the acoustical value of any mufiler so tested and compared.

As explained above, the detachable, slidable pipe 15 carrying the input microphone 17, is detachably connected to the muffler 14 through the sliding telescopic pipe connection therewith, as shown on the input side of the muffler at '16. The mufliler 14, on its outlet side 25, may have a similar telescopic, sliding, detachable connection with the pipe 20, as on the input side, whereby the parts are all readily adjustable, detachable and replaceable and, particularly, various muffler constructions or other sound-reducing means may be placed under comparative test in this apparatus to determine the effectiveness of the sound-reducing means. The microphones, likewise, are readily detachable and replaceable,

and the piping may be increased or reduced in length,

tachable, adjustable and telescopic pipe arrangement on the input side of the mufller 14 may be duplicated and utilized on the output side of the mufiler, if desired. Also, by reason of this construction, the extent or length of the pipes on either side of the muffler is made longitudinally adjustable.

Referring to the drawing, it will be observed that the pipes and the mufller and the loudspeaker and the antirefiection unit are all arranged in a straight line, and that the sound level meters 18 and 23 are preferably disposed in close proximity to one another so that a comparative reading may be readily taken from one convenient point.

From the foregoing it will be appreciated that sounds and noises of varying frequencies may be utilized in testing mufilers, and other sound-reducing equipment. That is, the sound created by the oscillator 10 or an internal combustion engine or other source of noise will have a specific frequency, and this same frequency will be tested before the sound enters the mufiier 14, and after entering the mutfier 14. As explained, the frequencies may be changed at will, the invention not being limited to any specific frequency.

I claim:

1. An apparatus comprising an audio oscillator, an amplifier for said oscillator and a loudspeaker connected to said amplifier, a pipe of substantially uniform diameter throughout its length, a housing for said loudspeaker associated with one end of said pipe, a mufller having an input side connected to the other end of said pipe, a microphone arranged in said pipe, a sound level meter connected to said microphone, a second pipe of substantially the same diameter as the first-named pipe connected to the output side of the mufiler, a microphone arranged in said second pipe, a sound level meter connected to said last-mentioned microphone, and an anti- References Cited in the file of this patent reflection sound unit disposed in the outlet of said sec- UNITED STATES PATENTS 0nd pipe by which the sound passes out of the system.

2. An apparatus according to claim 1 wherein said 2356478 Stryker 1944 2,394,461 Mason Feb. 5, 1946 muffler 1s detachably connected to the first-named pipe.

2,538,444 De Mars Jan. 16, 1951 3. An apparatus according to clalm 2 wherein said muflier is adjustably connected to each of the pipes for OTHER REFJIRENCES' longitudinal adjustment of the length of pipe on each New de i measures gain f hearing aid Radioside of the mufiler. 10 Electronics magazine, October 1948, page 41.

4. Apparatus according to claim 1 wherein the sound Measuring and recording muflier efliciency, Autolevel meters are in juxtaposed relation. motive Industries magazine, May 15, 1951, page 52. 

